I've asked the same question to many professional bloggers (I run a platform for bloggers).
"Did you use markdown to write blog posts". The answer was a big NO. Most of them use the default WordPress editor (now Gutenberg). Some of them use Google docs and other 3rd party tools.
As you said, markdown is mostly used by developers for their blogs. I haven't seen many professional bloggers not using it
I'm a developer and a blogger. I still don't use markdown :D while blogging